1. Field of the Invention.
The invention relates generally to a semiconductor fabrication method. More particularly, the invention relates to a process for forming low defect density heterojunctions in semiconductor devices.
2. Description of Related Art.
In advanced semiconductor devices, junctions between two different semiconductor materials are typically used to improve the performance of the device. These junctions, composed of layers of dissimilar semiconductor material, are known in the art as heterojunctions. The semiconductor materials used at the heterojunctions generally have non-equal band gaps and electron affinities.
A heterojunction has two pieces of semiconductor materials directly in contact with one another. It is often desirable to induce a specific type of bonding at the heterojunction. For example, forcing the heterojunction to have indium antimonide (InSb)-like bonding at an aluminum antimonide (AlSb) to indium arsenide (InAs) heterojunction for improved electron mobility, as described in G. Tuttle, H. Kroemer and J. English, “Effects of Interface layer sequencing on transport properties of InAs/AlSb quantum wells”, J.A.P.- Vol. 67, No. 6, 15 Mar. 1990, pp. 3033. Another example is forcing the heterojunction to have InSb-like bonding in a gallium antimonide (GaSb) to indium arsenide (InAs) superlattice grown on a GaSb substrate for lattice strain compensation.
A common method for achieving this desired bonding is using Molecular Beam Epitaxy (MBE) for growth. One advantage of using MBE growth is low temperature processing, which minimizes out-diffusion and autodoping in the semiconductor material. Another advantage is the precise control of doping profiles and deposition thickness that MBE allows. The MBE process can be used to control the bonding of group III-V materials. For example, the MBE process can be used to produce a GaSb to InAs heterojunction with InSb bonding, as opposed to GaAs bonding, at the heterojunction.
The prior art process of forming a AlGaSb to InAs heterojunction includes the following steps. First, AlGaSb is deposited on a base substrate. During the growth of AlGaSb, the aluminum (Al) and gallium (Ga) fluxes are stopped, and the surface is soaked with the antimony (Sb) flux. This covers any exposed aluminum or gallium atoms with an antimony atom, and any excess antimony should re-evaporate under typical MBE growth temperatures and fluxes. Next, the antimony flux is stopped, and a monolayer of indium is deposited. This indium monolayer is bonded to the antimony atoms on the surface of the substrate. After the deposition of the monolayer of indium, the indium flux is continued and a suitable arsenic flux is initiated, resulting in the growth of InAs. This method of forming the heterojunction is called “forcing InSb-like bonding”.
This procedure of interrupting growth, depositing a monolayer of the desired group III material, and then continuing growth has been discussed in the prior art. Unfortunately, this procedure causes defects at certain heterojunctions. For example, forcing InSb-like bonding at an InAs to AlGaSb heterojunction has resulted in tiny oval defects 113 nucleated at the heterojunction, as shown in FIG. 1. Meanwhile, forcing InSb-like bonding at an AlSb to InAs heterojunction does not result in nucleated defects. The tiny oval defects 113, shown in FIG. 1, formed at the InAs to AlSb heterojunction, have been correlated with excess leakage currents between metal gates (e.g. Schottky gates) and channels. If the metal gate crosses one of these defects, excess leakage from the gate to the channel of the transistor occurs. Hence, with an increasing demand for improved semiconductor fabrication methods, there remains a continuing need for a process for forming semiconductor materials with a low defect density.